Conventional ammonia synthesis processes use fossil fuels such as a natural gas as a hydrogen source that is a raw material for synthesis. Therefore, a rise in the ammonia production cost due to a steep rise in fossil fuel prices as well as environmental load due to carbon dioxide (CO2) emissions have been a problem.
As a method for producing hydrogen without using fossil fuels, there are an electrolytic method using electrolysis of water, a photocatalytic degradation method for water by solar light, and a thermochemical method using solar heat and nuclear energy.
The Haber-Bosch process has been employed as a method for synthesizing ammonia using hydrogen. Here, the Haber-Bosch process is a method for synthesizing ammonia from a hydrogen gas and a nitrogen gas, and produces ammonia by reacting a hydrogen gas with a nitrogen gas in the presence of an iron-based three-way catalyst.
The Haber-Bosch process is the mainstream of ammonia synthesis methods even at present because of its high synthesis efficiency, but the process has problems in that energy consumption and facility scale required are large because the process performs synthesis under high temperature and high pressure conditions, and that a large quantity of carbon dioxide (CO2) is emitted in obtaining a hydrogen gas by steam reforming of hydrocarbons.
A method that does not require a hydrogen gas has been developed as a method for solving the above problems, and patent documents relating to such an ammonia synthesis method are as follows.
First, an electrolytic ammonia synthesis apparatus disclosed in Patent Document 1 below is an apparatus for synthesizing ammonia from water and nitrogen, wherein the kind of steam to be supplied to an electrolytic bath and the means to agitate the electrolytic bath are devised. This electrolytic ammonia synthesis apparatus is characterized in that it is (1) an apparatus for synthesizing ammonia by supplying refined steam and N3− to a molten salt being an electrolytic bath, and includes (2) a means for supplying a gaseous component to the molten salt and agitating the molten salt by a rising stream of the molten salt containing the gaseous component, (3) an anode to generate an oxygen gas by oxidizing O2− produced by the reaction of steam, and (4) a cathode to generate N3− by the reduction of a nitrogen gas.
Here, the molten salt is at least one member selected from the group consisting of alkali metal halides and alkaline earth metal halides. In addition, the refined steam has a bubble diameter of 100 nm to 10 mm. Moreover, the refined steam is supplied so that the number of bubbles per 1 cm3 of the molten salt becomes 10 to 10000000.
Next, an ammonia synthesis apparatus disclosed in Patent Document 2 below is characterized by including a mesh-like or porous cathode to which a nitrogen gas is supplied, a nitride solid electrolyte layer on the cathode, a mesh-like or porous anode provided on the nitride solid electrolyte layer, and a catalyst layer provided on the anode for adsorption and dissociation of hydrogen, and in which nitrogen negative ions are electrochemically generated in the nitride solid electrolyte layer by applying to the anode a positive electric potential with respect to the cathode; atomic nitrogen is obtained at the anode by oxidizing the nitrogen negative ions; and ammonia is synthesized at the anode by reacting atomic nitrogen with atomic hydrogen to be adsorbed and dissociated on the catalyst layer.
Patent Document 1: JP 2009-84615
Patent Document 2: JP 2005-272856